Variability of the health effects of crystalline silica: Fe speciation in industrial quartz reagents and suspended dusts-insights from XAS spectroscopy

The speciation of Fe in bulk and suspended respirable quartz dusts coming from ceramic and Iron casting industrial processes is investigated through X-ray Absorption Spectroscopy (XAS), with the aim of contributing to unravel the variability of crystalline silica toxicity. Four different bulk industrial quartz powders, nominally pure quartz samples with Fe-contents below 200 ppm, and three respirable dusts filters were selected. Fe-speciation was determined in all samples through a coupled study of the XANES and EXAFS regions, operating at the Fe K edge. Fe speciation reveals common features at the beginning of the different production processes, whereas significant differences are observed on both respirable dusts and bulk dusts exiting from the production process. Namely, a common pollution of the raw quartz dusts by elemental Fe was evidenced and attributed to residuals of the industrial production of quartz materials. Moreover, the respirable samples indicate that reactivity occurs after the suspension of the powders in air. The gravitational selection during the particle suspension always allows to clearly discriminate suspended and bulk dusts. On the basis of the obtained results, an apparent spectroscopic discrimination is provided between the raw materials used in the considered industrial processes, and those effectively breathed from workers. In particular, an amorphous FeIII oxide, with unsaturated coordination sphere, can be related to silica reactivity (and health consequences).